1. Field of the Invention
The present invention relates to an electronic device, such as a laminated ceramic capacitor and a laminated ceramic inductor or the like, and a manufacturing method thereof, and in particular, to a chip-like shaped electronic device and a manufacturing method thereof, which has an improved structure in external electrodes of conductive films which are formed on surfaces of an element assembly (a bare or naked chip body) of the electronic device and improved in the formation thereof.
2. Description of Related Art
As an electronic device of the laminated type, a laminated ceramic capacitor and a laminated ceramic inductor are representative, however, there are many other compound elements, such as a LC device which combines those devices together.
With a laminated ceramic capacitor being a representative example of the laminated electronic device, a large number of internal electrodes in the form of a layer are laminated or piled up through ceramic layers, constitute circuit elements of the electronic device as a capacitor, and also extend to end surfaces of the element assembly of the electronic device. On the end surfaces of the element assembly of the electronic device, onto which the internal electrodes are extended, external electrodes of conductive film are formed.
In a general method for manufacturing the laminated ceramic capacitor, as an example thereof, a ceramic slurry of dielectric ceramic powder which is dispersed in an organic binder is formed into a shape of a sheet so as to form a ceramic green sheet, and on that ceramic green sheet is printed a pattern(s) of the internal electrodes of a conductive paste by a screen printing method, or the like. Then, the ceramic green sheets, on each of which is printed the internal electrode pattern(s), are piled up, and further at both sides of it are also piled up a plurality of the ceramic green sheets on which no internal electrode pattern is printed. The element assembly which is obtained in this manner is cut into a chip-like shape, thereby exposing the internal electrodes at the end surfaces thereof, and is baked. Then, on the end portions of the baked element assembly of the electronic device is pasted the conductive paste to form the external electrodes being baked thereafter, thereby completing the laminated chip capacitor.
Further, in another manufacturing method of the capacitor of the laminated ceramic type, before baking the ceramic element assembly of the electronic device, the conductive paste is applied on the end portions thereof in advance, and thereafter, the ceramic element assembly of the electronic device, as well as the applied conductive paste are baked, simultaneously. Furthermore, as the method for obtaining the element assembly of such an electronic device, there is also employed a so-called slurry built method, in which the ceramic paste and the conductive paste are painted one by one, other than the so-called sheet method using the ceramic green sheet.
For such electronic devices, including the laminated ceramic capacitor as a representative one thereof, there are the requirements of a large capacitance or high inductance and so on, while reducing the size thereof.
FIG. 3 shows an example of such the laminated ceramic capacitor. Inside of the element assembly 1 of the electronic device, i.e., a laminated body of ceramic layers 2, there are provided two (2) sets of the internal electrodes 3, 4 opposing each other mutually, and two (2) sets of the internal electrodes 3, 4 extend outside to the end surfaces of the element assembly 1 of the electronic device, which surfaces oppose each other. Upon both end portions of the element assembly 1 of the electronic device are formed the external electrodes 5, 5 of the conductive film, and those external electrodes 5, 5 are electrically connected to the two (2) sets of the internal electrodes 3, 4, respectively.
The chip-like shaped electronic device, represented by the laminated ceramic capacitor, is mounted on a circuit board or the like, and the external electrodes thereof are connected by soldering. At this instance, for the external electrodes, there are required a good solderability, a strong mechanical shear strength (bonding or adhesion strength) after soldering, and a good electrical conductivity. Therefore, the external electrodes 5, 5 are constructed of a plurality of metal layers, not just a single layer.
FIG. 4 shows the metal layers of the external electrodes 5, 5. First of all, upon the surface of the element assembly 1 of the electronic device, an anchor layer 6 is formed, which layer shows a good bonding or adhesion property upon the surface of the element assembly 1 and a good electrical conductivity with the internal electrodes 3, 4 (see FIG. 3). Upon the anchor layer 6, there is formed a solder-resistant layer 7 that shows a property against a so-called bitten by solder, i.e., the metal particles of a metal layer is reluctant to flow out into the melting solder. Furthermore, upon the solder-resistant layer 7, there is formed a solder-wettable layer 9 which has a good wettability with the molten solder.
As a means for forming the above-mentioned external electrodes 5, 5, there are known two types of methods, such as a wet process and a dry process, roughly classifying them.
With the wet process, the external electrodes 5, 5 are formed by means of printing and baking of the conductive paste or plating, and so on. In this wet process, in general, the mentioned anchor layer 6 is formed of a printed layer of Ag and the solder-resistant 7 of a plated layer of Ni, and further, the solder-wettable layer 9 of a plated layer of solder.
With the dry process, the external electrodes 5, 5 are made by forming the conductive film on the end portions of the element assembly 1 of the electronic device, by means of a vacuum evaporation or sputtering and so on. With this dry process, in general, the above-mentioned anchor layer 6 is formed of a film of Cr or a Cr alloy, the solder-resistant layer 7 of a film of Ni or Ni alloy, and further, the solder-wettable layer 9 is formed of a film of Ag, Sn or a Sn alloy.
In the wet process of the former, however, there are drawbacks that the external electrodes 5, 5 are difficult to form with a high accuracy because of the narrow pitches thereof, and further, the reliability is reduced since a plating liquid enters into the inside of the element assembly 1 of the electronic device during the plating thereof. Therefore, currently, the dry process is prevailing gradually.
However, when the external electrodes are formed by the dry process mentioned above, the bonding property between the solder-resistant layer 7 and the solder-wettable layer 9 is not so good, thereby, there is another drawback that the boundary surface between the solder-resistant layer 7 and the solder-wettable layer 9 is easily exfoliated or peeled in a test of the shear strength thereof.
Further, when the solder-wettable layer 9 is formed out of a metal material having a low melting or fusing point, such as Sn or solder, if the metal forming the solder-wettable layer 9 is sparse or non-dense, spottedly, the melting solder does not expand over the surfaces of the external electrodes 5, 5, when soldering. Under such a condition, the solder not only shows a bad wettability but also water or moisture within the air enters into the inside of the element assembly 1 of the electronic device, thereby causing the corrosion of the conductive body in its inside and also the short-circuiting thereof.